(1) Field of the Invention
The present invention relates to a coated ceramic member having excellent heat resistance, thermal shock resistance and corrosion resistance, as well as to a process for production of said member. More particularly, the present invention relates to a coated ceramic member having excellent durability under severe conditions, for example, in a high-speed combustion gas, as well as to a process for production of said member.
(2) Prior Art
As the high-temperature structural material used at high temperatures under severe conditions, ceramic materials such as silicon nitride, silicon carbide and the like have hitherto been known. As the ceramic material of good heat resistance, there is known, for example, a sintered silicon nitride member having a zirconia coating film formed thereon, disclosed in Japanese Patent Application Kokai (Laid-Open) No. 72582/1987.
Such silicon nitride or silicon carbide is a high-temperature-resistant material; however, it often thins owing to the high-temperature oxidation, high-temperature corrosion, etc. Hence, with respect to the member which is exposed to a high-speed combustion gas, there is an important task of improved oxidation resistance for improved durability.
In order to satisfy this task, there was proposed a method of forming an oxide ceramic film on the surface of a base material. In the thus-obtained member comprising a base material and an oxide ceramic film simply formed thereon, when the member is used at high temperatures for a long period of time, the difference in thermal expansion coefficient between the base material and the oxide ceramic film invites a very large stress-induced strain at their boundary, allowing the oxide film to peel from the base material easily.
In order to solve this problem, the present applicant proposed, in Japanese Patent Application Kokai (Laid-Open) No. 238859/-1993, a coated ceramic member comprising a base material and an oxide film formed thereon, wherein the oxide film consisted of an underlayer, an intermediate layer and a surface layer all formed by plasma spraying and wherein the thermal expansion coefficients of the three layers were made larger in the order of the underlayer, the intermediate layer and the surface layer.
The present applicant made further study and found out the following. The oxide film formed by plasma spraying was not sufficiently dense, inviting the oxidation of the base material; as a result, an oxidized layer generates between the base material and the underlayer, reducing the adhesivity between them and allowing the member to have inferior durability.